As an apparatus for performing cooling in a low-temperature range of minus several tens of degrees, a two-stage refrigeration apparatus including a high-stage refrigeration cycle being a refrigeration cycle device for circulating a high-temperature-side refrigerant and a low-stage refrigeration cycle being a refrigeration cycle device for circulating a low-temperature-side refrigerant has been used. One example of the two-stage refrigeration apparatus has a multistage configuration in which the low-stage refrigeration cycle and the high-stage refrigeration cycle are connected by a cascade condenser configured to allow a low-stage-side condenser in the low-stage refrigeration cycle and a high-stage-side evaporator in the high-stage refrigeration cycle to exchange heat with each other.
One example of such a two-stage refrigeration apparatus is the one in which when a low-stage-side compressor in the low-stage refrigeration cycle is inactive, a high-stage-side compressor in the high-stage refrigeration cycle is driven (see, for example, Patent Literature 1). In this two-stage refrigeration apparatus, during defrosting operation, the low-stage-side condenser in the low-stage refrigeration cycle is cooled by cooling a cascade heat exchanger by the evaporator in the high-stage refrigeration cycle to suppress a pressure rise inside the low-stage refrigeration cycle.
Another example of the refrigeration apparatus is the one in which, in the low-stage refrigeration cycle, a cooling pipe is connected through a collector disposed between the cascade condenser (low-stage-side condenser) and the cooler and a refrigerating machine and the cooling pipe are connected by a pipe (see, for example, Patent Literature 2). In this refrigeration apparatus, at the time the operation of the refrigeration apparatus is stopped, the refrigerating machine is operated, the cooling pipe is cooled, the refrigerant gas inside the collector is cooled, and the gas pressure of the refrigerant flowing in the low-stage refrigeration cycle is reduced.